net: ipv6: IPv6 environment variable cleanup

Fix "setenv gatewayip6".

Synchronize IPv6 local variables with environment variables
in netboot_update_env()

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
diff --git a/include/env_flags.h b/include/env_flags.h
index 7de58cc..d785f87 100644
--- a/include/env_flags.h
+++ b/include/env_flags.h
@@ -71,7 +71,7 @@
 #define NET6_FLAGS \
 	"ip6addr:s," \
 	"serverip6:s," \
-	"gatewayip6:s"
+	"gatewayip6:s,"
 #else
 #define NET6_FLAGS
 #endif